1. Field of the Invention
The invention concerns the control of an electric motor or any other electrical driving and/or generating device (transformer, etc.) using current loop control and employing an "H bridge" type switch mode amplifier.
2. Description of the Prior Art
One example of such an amplifier is shown in FIG. 1 in which T1, T2, T3 and T4 are solid-state switches (usually transistors), M is the electric motor in question (or one phase of the motor in the case of a multiphase motor) and I is the current in the motor. A reverse biased diode is connected to each switch. The upper and lower terminals of the H bridge are connected to maximum potentials, respectively.
To control the electric device M at the center of the bridge a control logic unit C determines according to the measured current and the instantaneous value of the set point (or control) current the setting for each of the four switches and the times for which these settings are maintained.
There are two conventional ways to control these bridges:
either so-called "diagonal" control in which the switches operate in diagonally opposite pairs expressed in logical notation as T1=T3=T2=T4 (Ti is the logical complement of Ti, Ti=1 if switch i is on, Ti=0 if switch i is off),
or so-called "freewheel" control from the top or the bottom: in the case of freewheel control from the bottom, for example, there are two forms of the control logic dependent on the direction of the control current: T1=0, T2=1, T3=T4 (negative current), and T1=T2, T3=1, T4=0 (positive current).
If T3=1 (negative current) of T2=1 (positive current) no potential is applied to the motor which therefore "freewheels", explaining the name given to this mode of control.
These two types of control are described in U.S. Pat. No. 4,581,565 which first describes the diagonal control mode and its drawbacks and then discloses, for eliminating those drawbacks, a special H bridge and a specific embodiment of the freewheel control mode.
The diagonal control mode involves switches in constant duration cycles: during a first part of this cycle the switches of one diagonal are turned on (the other diagonal being off) and then during a second part of this cycle complementary to the first part the switches of the other diagonal are turned on (and those of the first diagonal are turned off). The fraction obtained by dividing the duration of the first part by the total cycle duration is chosen according to the instantaneous difference between the set point current and the measured current, this measured current being the current flowing through the motor. U.S. Pat. No. 4,581,565 teaches the following measures to alleviate the drawbacks of this type of control: firstly, measuring the current flowing between the maximum and minimum potentials, instead of the current flowing through the motor, and, secondly, employing cycles of variable duration, applying the voltage difference between the maximum and minimum potentials to the motor if the measured current is less than the set point current and selecting freewheel mode for a specified duration immediately after the measured current exceeds the set point.
Diagonal switching provides a current loop which operates perfectly in all configurations of the motor, whether it is in fact operating as a motor (FIG. 2a) or as a generator (FIG. 3a), because the potential is applied to the motor at all times, in one direction or the other. The drawback of this mode of operation is that it creates a high level of current ripple (this is also related to the switching frequency) and therefore losses due to the Joule effect and generates high iron losses even when the motor is stationary (null current set point).
On the other hand, switching with freewheeling at the top or the bottom of the bridge provides a current loop which limits current ripple and iron losses (no voltage is applied to the motor when it is freewheeling) which leads to a slower variation in the current (FIG. 2b). However, in practice the current is not controlled during phases in which the motor operates as a generator, i.e. there are situations in which runaway of the motor can occur until the current becomes saturated (see FIG. 3b). One way to avoid this saturation (see previously mentioned U.S. Pat. No. 4,581,565) is to turn off all the transistors of the bridge for a specific time, but there is then no control over the current: there is thus a transient phase during which the current loop is no longer functional, which causes problems if this loop is integrated into a more general system. Note also (see FIG. 2b) that the small decrease in the current under steady state conditions (which reduces the amplitude of the ripple as compared with FIG. 2a) is achieved at the cost of a slower decrease than in FIG. 2a if the set point is suddenly reduced.
An object of the invention is to alleviate the aforementioned drawbacks of both types of control mode and is directed to a control method procuring a low level of ripple in the case of a steady state set point current combined with a fast variation in the instantaneous current in the event of a sudden variation (in particular reduction) of the set point current, with no risk of uncontrolled runaway of the electric device under current overload conditions, and with no risk of premature wear of the electric device.